Instability of buoyant diffusion flames

Buoyant jet diffusion flames are known to exhibit large scale vortical flow structures strongly interacting with flame structures. In the present work, the formation and evolution of coherent flow structures is studied in a methane/ air coflow arrangement. This is accomplished by utilizing visualization techniques (planar laser induced hydroxyl fluorescence and Mie-scattering) and Laser Doppler Velocimetry. A striking repeatability and correlation of evolving coherent structures of the air co-flow and the reaction zone is observed. In the transitional region, flow and flame structures oscillate at very pure frequencies ranging from 10–15 Hz. A local absolutely unstable velocity profile close to the burner rim seems to be responsible. Self-excited axisymmetric wavelike structures propagate up- and downstream of this location. We study the influence of the exit velocities and the type of coflowing oxidizer (air or oxygen) on the location of transition to periodic flow structures and related frequencies. Conditional averages of image and velocity data are employed to describe the evolution of coherent flow structures and their interaction with flame structures.The authors wish to thank the Deutsche Forschungsgemeinschaft for financial support under contract Kn 118/22-2.